BUG: Fix 57735 (#59335)

* Revert "CLN: Remove special cases in indexing ops (#52063)"

This reverts commit 8e456d3.

* remove old comments, add test

* use better test name

* Update pandas/tests/indexing/test_loc.py

Co-authored-by: Matthew Roeschke <[email protected]>

* check for empty index first

* assert assign to empty does not change frame

* format

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: matiaslindgren

pandas/core/indexing.py

@@ -810,8 +810,11 @@ def _maybe_mask_setitem_value(self, indexer, value):
 
                 if is_scalar_indexer(icols, self.ndim - 1) and ndim == 1:
                     # e.g. test_loc_setitem_boolean_mask_allfalse
-                    # test_loc_setitem_ndframe_values_alignment
-                    value = self.obj.iloc._align_series(indexer, value)
+                    if len(newkey) == 0:
+                        value = value.iloc[:0]
+                    else:
+                        # test_loc_setitem_ndframe_values_alignment
+                        value = self.obj.iloc._align_series(indexer, value)
                     indexer = (newkey, icols)
 
                 elif (
@@ -827,8 +830,11 @@ def _maybe_mask_setitem_value(self, indexer, value):
                         indexer = (newkey, icols)
 
                     elif ndim == 2 and value.shape[1] == 1:
-                        # test_loc_setitem_ndframe_values_alignment
-                        value = self.obj.iloc._align_frame(indexer, value)
+                        if len(newkey) == 0:
+                            value = value.iloc[:0]
+                        else:
+                            # test_loc_setitem_ndframe_values_alignment
+                            value = self.obj.iloc._align_frame(indexer, value)
                         indexer = (newkey, icols)
         elif com.is_bool_indexer(indexer):
             indexer = indexer.nonzero()[0]
@@ -2389,7 +2395,7 @@ def ravel(i):
                         new_ix = Index([new_ix])
                     else:
                         new_ix = Index(new_ix)
-                    if ser.index.equals(new_ix):
+                    if not len(new_ix) or ser.index.equals(new_ix):
                         if using_cow:
                             return ser
                         return ser._values.copy()

pandas/tests/indexing/test_loc.py

@@ -3272,3 +3272,10 @@ def test_loc_index_alignment_for_series(self):
         df.loc[:, "a"] = other
         expected = DataFrame({"a": [999, 200], "b": [3, 4]})
         tm.assert_frame_equal(expected, df)
+
+    def test_loc_reindexing_of_empty_index(self):
+        # GH 57735
+        df = DataFrame(index=[1, 1, 2, 2], data=["1", "1", "2", "2"])
+        df.loc[Series([False] * 4, index=df.index, name=0), 0] = df[0]
+        expected = DataFrame(index=[1, 1, 2, 2], data=["1", "1", "2", "2"])
+        tm.assert_frame_equal(df, expected)